The present invention is directed generally to an electrode material for fluorescent lamps, and more particularly to an emissive mixture having a formula (A1-x Cax)6 (Ta1-yWy)2 O11+y, where A is barium or barium and strontium, coated on fluorescent lamp cathodes.
The standard emissive coating currently used on a majority of electrodes of commercial fluorescent lamps contains a mixture of barium, calcium, and strontium oxides (xe2x80x9cthe triple oxide emissive mixturexe2x80x9d). Since these oxides are highly sensitive to CO2 and water, they are placed on the lamp electrodes initially as a mixture of barium, calcium and strontium carbonates in a slurry suspension containing a binder and a solvent. The mixture of carbonates is then xe2x80x9cactivatedxe2x80x9d during the manufacturing process by resistively heating the electrodes until the carbonates decompose, releasing CO2 and some CO, and leaving behind the triple oxide emission mixture on the lamp electrode. It is believed that barium, in some form, is primarily responsible for the electron emission. It is also known to add a small amount of Al, Hf, Zr, Ta, W and Th dopants to the triple oxide emissive mixture, as discussed in U.S. Pat. No. 3,563,797 to Young.
However, the triple oxide emissive mixture suffers from several disadvantages. First, the xe2x80x9cactivationxe2x80x9d requires an undesirably high temperature to convert the carbonates to oxides. Second, the conversion of carbonates to oxides undesirably releases CO2 and some CO. Third, lamps having electrodes coated with the triple oxide emissive mixture have a rather short operating lifetime.
It has been previously suggested in U.S. Pat. No. 4,031,426 to Kern, incorporated herein by reference in its entirety, to substitute the triple oxide emissive mixture with barium tantalate emissive mixtures having various barium to tantalum ratios. According to U.S. Pat. No. 4,031,426, the barium tantalate emissive mixtures may be xe2x80x9cactivatedxe2x80x9d in less time and at a lower temperature than the triple oxide emissive mixture. Furthermore, lamps having electrodes coated with the barium tantalate emissive mixtures have a longer operating lifetime than the lamps with the triple oxide emissive mixture. The preferred barium oxide to tantalum pentoxide ratio in U.S. Pat. No. 4,031,426 is 5:1, resulting in an emissive mixture having the following composition: Ba5Ta2O10. Emissive mixtures having barium oxide to tantalum pentoxide ratios greater or less than 5:1 were reported in this patent as having a lower electron emission.
However, a fluorescent lamp containing the barium tantalate emissive mixture has a somewhat inferior efficacy compared to the triple oxide emissive mixture. The present invention is directed to overcoming or at least reducing the effects of one or more problems set forth above.
One embodiment of the present invention provides a composition of matter, comprising (A1-x Cax)6 (Ta1-y Wy)2 O11+y, wherein A comprises one of barium or a combination of barium and strontium, 0xe2x89xa6x less than 0.5, 0xe2x89xa6y less than 1, and at least one of x or y is greater than zero.
Another embodiment of the present invention provides a method of making an emissive mixture comprising a composition of matter having a formula (A1-x Cax)6 (Ta1-y Wy)2 O11+y, the method comprising determining a desired ratio of a mean lamp efficacy to a mean number of lamp starts by determining the amount of Ca and W to be present in the emissive mixture, mixing predetermined amounts of starting materials based on the determination of the desired ratio to form an emissive mixture and coating the emissive mixture on an electrode, wherein A comprises one of barium or a combination of barium and strontium, 0xe2x89xa6x less than 0.5, 0xe2x89xa6y less than 1 and at least one of x or y is greater than zero